1. Field of the Invention
The present invention relates to an electrostatic lens used in electron microscope, or electron or ion-beam-utilizing apparatus, and to a method for producing it.
2. Related Background Art
An electrostatic lens has been used as an electron-gun lens in electron microscope or as a focusing lens in ion-beam apparatus.
FIG. 5 shows a typical example of conventional cylindrical electrostatic lens and a ray diagram thereof.
In FIG. 5, reference numeral 24 designates a position of electron gun or crossover. An electron beam emerges at a divergence angle from the electron-gun or crossover position 24. A first electrode 25 is earthed to earth 30, a second central electrode 26 is connected to a power source 19 in a negative potential forming a lens electric field, and a third electrode 27 is earthed to the earth 30.
The electrostatic lens with the structure shown in FIG. 5 is generally called as einzel lens, which is composed of three electrodes 25, 26, 27. The electrostatic lens is thus constructed of a plurality of cylinder or disk electrodes, such as the electrodes 25, 26, 27. When a voltage is applied from the power source 19 to the second electrode 26, an electron beam is focused on a sample table 29. FIG. 6 shows a potential distribution of this electrostatic lens. As shown in FIG. 6, a potential peak (-Vp) appears at the second electrode 26 and the potential decreases to zero toward either of the first and third electrodes 25, 27. The smoother the shape of potential distribution in FIG. 6 the smaller the lens aberration as long as the focus position is fixed.
FIG. 7 shows another electrostatic lens. The electrostatic lens shown in FIG. 7 provides an asymmetric distribution of axial potential to reduce aberration, which is called as asymmetric einzel lens. Similarly as in FIG. 5, a first electrode 31 is earthed, a second central electrode 32 is kept in a negative potential forming a lens electric field, and a third electrode 33 is kept earthed. In FIG. 7, the electrodes 31, 32, 33 are held by a ceramic holder 34 for insulation.
In the electrostatic lens shown in FIG. 5 or in FIG. 7, an electron beam emerges from the electron-gun or crossover position 24 (FIG. 5) and focuses at the sample table 29 through the electrostatic lens.
In the example shown in FIG. 7, the electrodes 31, 32, 33 in the electrostatic lens are held and fixed by the ceramic holder 34 (insulator). The ceramic holder 34 keeps the positional accuracy between the electrodes 31, 32, 33. The electrodes 25, 26, 27 are also held and fixed by an unrepresented ceramic holder in the electrostatic lens shown in FIG. 5.
As described above, either electrostatic lens shown in FIG. 5 or in FIG. 7 includes a plurality of (for example three) electrodes held and fixed by a ceramic holder. However, in order to hold and fix a plurality of electrodes by a ceramic holder, assembling is not easy with troublesome work for accurately positioning the electrodes with respect to the center axis of electron beam.
Also, in case of the electrostatic lens shown in FIG. 7, each electrode is formed in a complicated shape in order to prevent electrification in the ceramic holder and to reduce lens aberration. If the electrodes are formed in complicated shape for electrification prevention in the ceramic holder, the ceramic holder must also have a complicated shape matching the electrodes, which makes the production difficult and increases the size of entire lens.